The present invention relates to a method and apparatus for controlling a DC-DC converter having a half-bridge configuration.
FIG. 5 is a block circuit diagram of a conventional DC-DC converter. As shown in FIG. 5, the conventional DC-DC converter includes a first series circuit including metal-oxide-semiconductor field effect transistors (MOSFETs) 1, 2 connected in parallel to a DC power supply 10, a second series circuit including a capacitor 3 and a primary winding 6 of a transformer 9 and connected in parallel to the MOSFET 2, a snubber capacitor 4 connected in parallel to the MOSFET 2, and a rectifying and smoothing circuit 5 including a diode 11 connected to a secondary winding 7 of the transformer 9 and a diode 12 connected to another secondary winding 8 of the transformer 9. A first output voltage detector circuit 16 detects an output voltage VO. A triangular wave generator circuit 14 generates a triangular wave signal, the frequency thereof changes corresponding to the difference between the output voltage VO and a reference output voltage. A comparator 15 compares the frequency of the triangular wave signal with the output from an on-off ratio setting circuit 13. A driver circuit 18 switches the MOSFETs 1, 2 on and off alternately at a fixed on-off ratio of 50%.
As described above, the conventional DC-DC converter shown in FIG. 5 controls the output voltage thereof by changing the switching frequency FS of the MOSFETs 1 and 2 at a fixed on-off ratio. Since the DC-DC converter described above is a general current-resonation-type one and since the operations thereof are also general, detailed descriptions on the conventional DC-DC converter will be omitted.
In the conventional DC-DC converter, the exciting inductance of the transformer should be low enough to prevent the switching frequency from increasing greatly. FIG. 6 is a set of curves relating the switching frequency FS for the DC power supply voltages Ed of 100 V and 400 V and the on-off ratio D with the output power PO. In the conventional DC-DC converter, although the switching frequency changes to some extent depending on the load condition and the input voltage from the DC power supply, the on-off ratio D shows almost no change.
FIG. 7 is a wave chart describing the currents IQ1 and IQ2 flowing respectively through the MOSFETs 1, 2, and the currents ID11 and ID12 flowing respectively through the diodes 11 and 12 connected to the secondary side of the transformer under the rated load condition. FIG. 8 is a wave chart describing the currents IQ1 and IQ2 flowing respectively through the MOSFETs 1, 2, and the currents ID11 and ID12 flowing respectively through the diodes 11 and 12 connected to the secondary side of the transformer under the light load condition.
As a result of reducing the exciting inductance of the transformer to prevent the switching frequency from increasing greatly under the light load condition, high exciting currents flow into the transformer under the rated load condition as well as under the light load condition as the currents IQ1 and IQ2 in FIGS. 7 and 8 indicate. The exciting currents cause reactive currents, which further cause loses across the impedance in the circuit such as the on-resistance of the MOSFETs and the wiring resistance of the transformer. Due to the loses caused, the conversion efficiency of the DC-DC converter is low under the light load condition.
In view of the foregoing, it would be desirable to provide a method and apparatus for controlling a DC-DC converter, that facilitates preventing the switching frequency from increasing under a light load condition and improving the conversion efficiency of the DC-DC converter.